1. Field of the Invention
This invention relates to an improved method for the production of needlelike .alpha.-alumina, .theta.-alumina, mullite, and sillimanite. More particularly, it relates to a method for efficient production of needlelike .alpha.-alumina, .theta.-alumina, mullite, and sillimanite by using as starting material therefor needlelike alumina precursor particles or needlelike .gamma.-alumina polycrystalline particles obtained by thermally decomposing the precursor particles mentioned above.
2. Description of the Prior Art
In the conventional methods for the production of needlelike alumina particles, the method which effects deposition of alumina whiskers on the surface of an alumina substrate by heating and decomposing aluminum chloride in a vacuum or by sweeping molten aluminum with a hydrogen gas containing a minute amount of steam at a temperature near 1,250.degree. C. [Journal of Applied Physics, Vol. 33, pages 33 to 39 (1962)] and the method which comprises thermally decomposing fine particles of Al(NH.sub.4)(CO.sub.3)(OH).sub.2 as a needlelike alumina precursor [Japanese Patent Publication No. HEISEI 2-624851 have been renowned.
The former method is barely capable of producing alumina whiskers deposited on an alumina substrate in an extremely small amount of laboratory scale. The conditions of production are extremely difficult to control. The products obtained by this method have poor reproducibility in terms of kind and quality. The production cannot be obtained on a commercial scale. As respects the equipment for the operation of this method, a device for vacuum treatment necessitated for the purpose of performing a heat treatment in a vacuum, a device for controlling the vacuum pressure, a device for refining a reaction gas and controlling the flow volume of the reaction gas, a device for accurate control of the temperature of a reaction chamber, etc. which invariably are expensive. The latter method is capable of producing needlelike particles of polycrystalline .gamma.-alumina but incapable of producing needlelike .alpha.-alumina single crystal particles which are useful for raw material for composite materials.
In the conventional methods for the production of needlelike particles of aluminum-containing oxide, the method which produces needlelike mullite particles by heat-treating and decomposing a clay mineral [journal of The Ceramic Society of Japan, Vol. 96, No. 11, pages 1081 to 1086 (1989)] has been renowned. This method is disadvantageous in respect that the production of mullite particles necessitates an additional step for separating and removing a matrix formed mainly of by-produced silica. Since this method gives rise to waste, it is undesirable from the standpoint of economization of resources and conservation of the environment.
In recent years, needlelike and fibrous ceramic materials are required in various fields. As things stand now, the raw materials which meet the requirement and which are produced on a commercial scale are whiskers and long fibers of rather limited kinds of substances such as silicon nitride, silicon carbide, and carbon. Particularly, successful cases using oxide ceramics are rare. But, needlelike .alpha.-alumina single crystal particles are actually in strong demand.
This invention satisfies the demand and aims to provide a method for producing needlelike .alpha.-alumina single crystal particles with a simple apparatus by an easy procedure with high repeatability on an economic scale without entailing occurrence of waste. This method is characterized by allowing selective production of single crystal particles of .theta.-alumina, mullite, sillimanite, etc. by altering the conditions to be used for the production.